Nuclear fuel bundles are held together by tie rods. These tie rods having threaded engagement with the upper tie plate and lower tie plate of the fuel bundle. Beside contributing to the overall nuclear steam generation of the fuel bundles, the tie rods function mechanically in two ways. First, they maintain the fuel bundles as a unitary assembly. Secondly, they permit disassembly of the fuel bundles for inspection of the individual fuel rods interior of the fuel bundle during reactor outages. Accordingly, there is disclosed an improved method and apparatus for assuring that the tie rods and tie plates remain locking the fuel bundle into a unitary assembly.
In order to understand this invention, the construction of a fuel bundle will first be discussed. Secondly, operation of the fuel bundle will be briefly set forth. Finally existing practice in the disassembly and inspection of such fuel bundles will be set forth. Once this has been done, the invention herein and its advantages can be fully understood.
Fuel bundles for boiling water nuclear reactors include a plurality of side-by-side vertically upstanding sealed fuel rods, these fuel rods containing the fuel. In so far as the mechanical integrity of the fuel bundle is concerned, the fuel rods are divided into two classes. First, there is a class of fuel rods that is merely captured between the upper and lower tie plate within the fuel bundle These are the "ordinary" rods. Secondly, there is a class of fuel "tie rods" that attached through threaded engagement to the lower tie plate and through a threaded nut to the upper tie plate. These are the so-called "tie rods" which in effect tie the fuel bundle into a unitary assembly
Over simplifying the construction of a nuclear fuel bundle, the lower tie plate functions to support the fuel rods and threads in engagement to typically eight tie rods. The upper tie plate--together with the lower tie plate--traps the ordinary fuel rods in vertical upstanding relation and ties to the tie rods at threaded nuts. Spacers are placed at regular vertical intervals to maintain designed spacing of the fuel rods for maximum nuclear efficiency. So-called water rods are placed at central locations in the fuel bundle and filled with water to improve the nuclear characteristics of the fuel bundle through improved neutron moderation. When all of these components are assembled together by securing the tie rods to the upper and lower tie plates, a nuclear fuel bundle which is a discrete separately handled unit of fuel for a boiling water nuclear reactor is created.
The fuel bundles must under no circumstance come apart during operation of the boiling water nuclear reactor. This being the case, elaborate precautions are taken to make sure that the threaded attachment of the tie rods does not come apart. The major precaution taken is the so-called locking tab at the top of the fuel bundle. These locking tabs fit over adjacent paired tie rods on the upper side of the upper tie plate and are held between the upper tie plate on the bottom and the fastening nuts on the top. The function of these locking tabs is relatively easy to understand.
First, the tie rods each have a slot at their respective upper end plugs. These slots mate with corresponding protruding nubs within the holes in the fastening tabs that extend around the tie rod end plugs. Remembering that each fastening tab fits around at least two tie rods, the tab itself cannot rotate. Further, when the nub fits into the slot in the tie rod, the tie rod cannot rotate relative to the locking tab and hence cannot rotate relative to the lower tie plate.
The upper tie plate is fastened to the threaded upper end of the tie rods. A nut is used which threads over the tie rod at the upper end and compresses the upper tie plate onto a spring between the end of the tie rod and a hole in the upper tie plate. Once this nut is in place, securing the nut against inadvertent rotation is required.
Securing the nut to the upper end of the tie rod occurs through strips or tangs coming up from the fastening tabs. These tangs are bent to fit against the sides of the hex nut utilized to fasten the upper tie plate. Since the fastening tabs cannot rotate, and the tangs from the fastening tabs contacting the sides of the hex nut prevent the nuts from rotation, the nuts fastening the upper tie plate are effectively locked in place. Thus, inadvertent disassembly of the tie rods, tie plates, and fuel bundle components held together by the tie rods and tie plates cannot occur.
All fuel bundles in boiling water nuclear reactors have a fuel bundle channel placed over the assembled fuel bundle. This channel may or may not be replaced with the nuclear fuel bundle; channels frequently are utilized for at least a second cycle, even though the fuel bundle within the channel has been replaced.
The fuel bundle channel is important when it comes to the operation of the fuel bundle. Specifically, the channel surrounds the fuel rods from the vicinity of the lower tie plate to the vicinity of the upper tie plate. It thus helps each fuel bundle to have an exclusive steam generating flow path which is separate from both the immediately surrounding core bypass region (which is liquid water moderator for improved nuclear performance) and the steam generating flow path through all remaining fuel bundles. In operation, water moderator enters the channel by passing through the lower tie plate and around the fuel rods. Water and generated steam leave the fuel bundle at the top of the channel typically by passage through the upper tie plate.
Boiling water nuclear reactors operate for specific periods of time. Thereafter, they are taken off line, depressurized, and serviced at intervals herein called "outages." During such outages, 1/3 to 1/5 of their respective fuel bundles are replaced. At the same time, inspection of other fuel bundles can be required to occur. And when such inspection occurs, the fuel bundles need to be disassembled. Given the locking tabs now in use, this disassembly is not convenient.
Once the fuel bundles have been within a nuclear reactor for a full cycle, they are the source of radiation. This being the case, disassembly and inspection of the fuel bundles must occur under a shielding water layer on the order of at least six feet of depth. Thus, all tools utilized function remotely under a water depth usually exceeding six feet.
In order to disassemble the fuel bundles, the upper tie plates must be removed. First, the tangs of the locking tabs in contact with the side of the hex nuts are bent out of contact with the sides of the nuts. Thereafter, the nuts are loosened. Finally, the locking tabs are lifted and discarded. Where the fuel bundle is reassembled after inspection, this process is reversed with the discarded locking tabs being replaced.
From the standpoint of reactor servicing, these required and laborious steps present two difficulties. This effort is time consuming, taking about 20 minutes. Reactors when off line are extremely expensive, costing hundreds of thousands of dollars per hour in lost utility revenue. Simply stated, time added in disassembly and reassembly is money lost in revenue.
Secondly, and more importantly, although all disassembly operations are done over a holding pool where any radiation is minimal, it is required by both regulation and safety that all exposure to such radiation be kept as low as reasonably achievable. Accordingly, the scheme for locking of the tie rods according to this invention has been developed.